Locking element for a polyaxial bone anchor, bone plate assembly and tool

ABSTRACT

A locking element is configured to be used with a polyaxial bone anchor, and is substantially cylindrical with a bottom side facing a head of the bone anchor, a top side opposite to the bottom side, and a surface portion with an external thread. The locking element has a first recess at the bottom side for accommodating at least a portion of the head, and a second recess at the top side for engagement with a driver. The second recess is located outside the center of the top side. The head has a portion with a largest diameter, and a first level L1 is defined by the highest point of a circle with the largest diameter drawn around the head, and a second level L2 is defined by the bottom of the second recess, L2 being at a level not higher than L1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, claims priority to and whollyincorporates by reference U.S. provisional patent application No.61/311,517 filed Mar. 8, 2010, US provisional patent application no,61/369,495 filed Jul. 30, 2010, and European patent application no.EP10158246.8 filed Mar. 29, 2010.

The invention relates to a locking element for a polyaxial bone anchorand to a bone plate assembly for the immobilization of bones or bonefragments or vertebrae and a tool used therefor. Particularly, theinvention relates to a bone plate assembly with a polyaxial couplingbetween the bone anchors of the bone plate assembly and a plate with anincreased range of angular motion and low profile.

U.S. Pat. No. 6,022,350 describes a bone fixing device comprising anelongate single-piece plate-shaped element receiving at least onebone-fastening screw which passes through an orifice formed in theplate-shaped element. The head of the screw includes an essentiallyspherical surface for bearing against a bearing surface of essentiallycircular cross-section in the bottom of the plate element. The devicefurther includes a plug for coming into clamping contact against saidscrew head to hold it in a desired angular position. The outside face ofthe plug includes a central tightening socket which is identical to thatin the screw head so that the same tightening tool can be used.

U.S. Pat. No. 5,531,746 describes a polyaxial screw plate assembly forimmobilization of vertebral bones including an elongate plate having aplurality of holes wherein the bottom portion of the holes has a curvedinterior surface which forms an annular lip for supporting asemi-spherical head portion of a bone screw. A coupling element in theform of a short threaded cylindrical piece having a concave bottom locksthe screw into the hole.

In addition, various bone plate assemblies are known which use bushingswith a spherical exterior surface which can pivot in holes with aspherical interior surface and which encompass a screw head to providepolyaxial angular adjustability of bone screws relative to the plate.Examples of such assemblies are known, for example, from WO 99/05968 andWO 00/04836.

While the known bone plate assemblies can provide polyaxial adjustmentof the bone anchors relative to the plate, there is still a need for animproved bone plate assembly which allows for a greater range of angularpositions, a lower profile of the plate-screw construct and a variety ofusage.

It is therefore the object of the invention to provide a low profilebone plate assembly and a locking element for a polyaxial bone anchorand a tool therefor which allows a polyaxial adjustment of the boneanchor and the plate with a wide range of angular motion with a simpledesign while simultaneously providing a high safety of use.

The object is solved by a locking element and a bone plate assemblyaccording to the claims of the present invention.

The bone plate assembly according to the invention allows a pivot angleof the bone anchor relative to the plate of up to around 35° withrespect to the vertical position. This corresponds to a motion cone ofup at least 60°. The position of the bone anchor relative to the platehas an angular stability due to the locking element. The locking can becarried out by application of low torque. With the locking element, thescrews are secured against pull-out. Different locking elements can beprovided to achieve either full locking or frictional locking or toallow free angulation while only preventing pull-out of the anchor.

In addition, the bone plate assembly is designed with a minimum ofnecessary parts and has a low profile. Due to the simple design and thesmall number of parts which are setting up the bone plate assembly, itis economical to manufacture.

The holes for the bone anchors in the plate member are designed suchthat bone anchors with or without a locking member can be used.Additionally, locking plugs can be provided to close a hole without theuse of a bone anchor, for example, if there are small bone fragments orfor ensuring stability against cracking.

Furthermore, the plate member may have offset holes which are offsetfrom a central longitudinal line for more variety of usage. The platemember can be designed to have a minimal bone contact area and can beused as a dynamic plate. Also, the plate member may be contoured toprovide a specific shape for specific clinical applications.

The bone plate assembly is suitable for various clinical applications.For example, due to the low profile design, the bone plate assembly issuitable for the application in areas with minimal soft tissue coveragelike the cervical spine or other small bones like the clavicle or thepelvis.

Further features and advantages of the invention will become apparentfrom the description of embodiments with reference to the accompanyingdrawings. In the drawings:

FIG. 1 shows a perspective view of a bone plate assembly according to afirst embodiment.

FIG. 2 shows a schematic exploded sectional view of the portion of thebone plate of FIG. 1 including a hole, a bone screw and a lockingelement.

FIG. 3 shows a schematic sectional view of a portion of the bone plateof FIG. 1 including a hole and an inserted bone screw.

FIG. 4 shows a schematic sectional view of a portion of the bone plateof FIG. 1 with an inserted bone screw and a locking element.

FIG. 5 shows a side view of the locking element of FIG. 4.

FIG. 6 shows a top view of the locking element of FIG. 4.

FIG. 7 shows a bottom view of the locking element of FIG. 4.

FIG. 8 shows a schematic sectional view of a portion of the bone plateof FIG. 1 with an inserted bone screw in a vertical position and aslightly modified locking element.

FIG. 9 shows the arrangement of FIG. 8 with the bone screw in a pivotedposition.

FIG. 10 shows a schematic sectional view of a portion of the bone plateof FIG. 1 including a second type of hole.

FIG. 11 shows a schematic sectional view of a portion of the bone plateof FIG. 1 including the second type of hole with an inserted bone screw.

FIG. 12 shows a perspective view from the top of a second embodiment ofthe bone plate assembly.

FIG. 13 shows a perspective view from the bottom of the bone plateassembly of FIG. 12.

FIG. 14 shows an enlarged schematic sectional view of a hole of the boneplate according to FIG. 12 sealed by a plug member.

FIG. 15 a) shows a schematic sectional view of a further embodiment ofthe bone plate assembly including the hole, the bone screw and amodified locking element, wherein the bone screw is in a straightposition.

FIG. 15 b) shows the bone plate assembly of FIG. 15 a) with the bonescrew in an angled position.

FIG. 16 shows an enlarged perspective view of the locking element of theembodiment according to FIGS. 15 a) and 15 b).

FIGS. 17 a) to 17 e) show a schematic sectional view of a portion of thebone plate with a polyaxial bone screw and a locking element,respectively, to illustrate a dimensional relationship between therecess for the driver and the head of the bone screw.

FIG. 18 shows a perspective view of a tool for inserting the lockingelement.

FIG. 19 shows an enlarged schematic sectional view of a portion of thetool of FIG. 18.

FIG. 20 shows a further enlarged schematic sectional view of a portionof the bone plate with a polyaxial bone screw, the locking element andthe tool.

FIGS. 21 a) to 21 d) show steps of using the tool.

A first embodiment of the bone plate assembly will now be described withreference to FIGS. 1 to 9. As can be seen in particular in FIGS. 1 and 2the bone plate assembly includes a plate member 1 which is in thisembodiment a substantially rectangular body with a top side 1 a, abottom side 1 b, short sides 1 c and long sides 1 d and a longitudinalaxis L. A plurality of first holes 2 extend through the plate memberfrom the top side 1 a to the bottom side 1 b. Second holes 3 can beprovided which are, for example, arranged between the first holes 2 alsoextend from the top side 1 a to the bottom side 1 b. However, the numberand arrangement of first and second holes can vary according to the sizeand shape of the plate member 1. The plate member may have only firstholes 2 and the second holes 3 can be omitted.

The bone plate assembly further comprises a plurality of bone anchors inform of bones screws 4 which can be inserted into the first holes 2and/or the second holes 3. The bone screw 4 has a threaded shank 41 witha tip and a head 42. The head 42 has a spherical exterior surfaceportion and a recess 42 a at its free end 42 b opposite to the shank 41for engagement with a screw driver. Other longitudinal bone anchors likepins, barbed or roughened nails can also be used.

As can be seen in FIGS. 1 and 2 a locking element 5 is provided forlocking the bone screw 4 in the first holes 2 as described below.

The structure of the first holes 2 will now be described with referenceto a first hole 2 shown in FIGS. 3 and 4. The first hole 2 comprises anopening 20 at the bottom side 1 b the inner diameter of which is largerthan the outer diameter of the threaded shank 41 of the bone screw sothat the threaded shank 41 can pass therethrough. The diameter is,however, smaller than the largest outer diameter d of the head 42 sothat the head 42 cannot pass therethrough. Adjacent the opening 20 ahollow seat portion 21 is provided which forms a socket for a pivotingmovement of the head 42. This allows insertion of the bone screw at anydesired angle. In the embodiment shown, the seat portion 21 isspherically-shaped with a radius that matches the radius of thespherically-shaped portion of the head 42. When the head rests in theseat portion, a part of the spherical section of the head projects outof the bottom side 1 b. The height h of the seat portion 21 is smallerthan the radius of the head 42 and preferably smaller than half of, morepreferably smaller than or equal to a quarter of the radius of the head42. Between the seat portion 21 and the top side 1 a a cylindrical bore22 with a portion 23 having an internal thread is provided. As shown inFIG. 3, in one exemplary aspect of the invention, this internal threadis a full thread, reaching to the top of the cylindrical bore 22. Theinner diameter of the bore 22 is larger than the inner diameter of theseat portion 21 and larger than the outer diameter of the sphericalportion of the head 42. In other words, the inner diameter D of the bore22 is larger than the outer diameter d of the head 42. By means of this,access to the head 42 with a screw driver is possible even at largepivot angles. For example, the inner diameter D of the bore 22 may, inone exemplary aspect of the invention, be approximately 1.2 to 1.7 timeslarger than the maximum outer diameter d of the head 42. However, thisexemplary aspect is for illustration only and is not a limitation on thescope of the invention. The threaded portion 23 can have any threadform, for example a metric thread. The thickness 1 of the plate member 1at the hole, that means the distance of the top side 1 a from the bottomside 1 b is smaller or equal to the largest outer diameter d of the head42.

The opening 20 widens in a conical portion 24 towards the bottom side 1b to allow even larger pivot angles of the bone screw 4.

The locking element will now be described with reference to FIGS. 1, 2and 4 to 7. The locking element 5 is substantially cylindrical and has atop side 5 a and a bottom side 5 b and a threaded exterior surfaceportion 51 which cooperates with the internally threaded portion 23 ofthe bore 22 of the plate member. The height of the locking elementcorresponds substantially to the depth of the bore 22 so that when thelocking element 5 is screwed into the bore 22 its top side 5 a issubstantially flush with the top side 1 a of the plate member.

As can be seen in particular in FIG. 4, the locking element is partiallythreaded. The non-threaded portion 51 a has an axial length which may beabout equal to or smaller than that of the threaded portion 51. Thelocking element can also be fully threaded.

On the bottom side 5 b the locking element 5 comprises preferably aspherically-shaped recess 52 which fits to the spherically-shapedportion of the head 42. The depth of the recess 52 can be equal to orlarger than the radius of the spherical portion of the head 42. By meansof this, the pressure exerted by the locking element 5 onto the head 42is smoothly distributed onto the head 42.

A recess 53 in form of a ring-shaped groove with a contour which allowsa form-fit engagement of a corresponding tool which is provided in theouter radial area is provided in the top side 5 a of the locking element5. As shown in FIG. 6, the contour of the groove in this embodiment hasa star-like shape. The central area of the top side 5 a is solid, i.e.is without a recess.

As can be seen in particular in FIG. 4, the depth of b of the recess 53is larger than the distance a of the deepest portion of the recess 52from the top side 5 a. Since the recess 53, which serves for engagementwith a driver, is in the outer area of the top side 5 a, the sphericalrecess 53 extending from the bottom side 5 b can have a large depthwhich results in an increased contact area between the locking element 5and the head 42. On the other hand, the seat portion 21 can be keptsmall to allow large angulation of the bone screw 4.

Furthermore, with the design described above the locking element has areduced height. As a consequence thereof, the overall height of the boneplate assembly can be kept small.

It is possible to provide different locking elements which differ intheir axial length or a different depth in the spherically-shaped recessto achieve either full locking of the head or a frictional locking.Frictional locking allows pivoting under application of an additionalforce which exceeds the frictional force between head and plate member.A further different locking element may have a length or a differentdepth in the spherically-shaped recess which allows a free pivotalmovement of the screw with the locking element only preventing pull-outof the screw.

The overall height of the bone plate assembly can be further downsizedby using a two-start thread for the threaded portions.

The design of the holes 2 and the looking element 5 are not restrictedto the specific embodiment shown. For example, the seat portion 21 needsnot to be spherically-shaped, but can have another shape such as a taperor even can be realized only by the edge of the opening 20. Between theseat portion 21 and the bore 22 transitional sections of the hole can bearranged provided they do not restrict the pivoting motion of the screwhead. The bore 22 is shown to be cylindrically-shaped. However, it couldalso be a conical bore. The locking element would then be adaptedthereto. Since only a relatively small clamping force is necessary tolock the angular position of the bone screw, other kinds of connectionsbetween the locking element and the bore can be used, for example, abayonet locking structure. Also, the orientation of the holes invertical direction needs not to be perpendicular to the surface but caninclude an angle with the normal onto the surface of the plate memberfor providing an initial angulation.

In FIGS. 8 and 9, the locking element 5 is slightly modified in such away that the recess 53 is closer to the outer edge than in the lockingelement 5 shown in FIG. 4. The recess 52′ is deeper than the recess 52of the locking element of FIG. 4 and has a flat portion near the topside 5 a. The profile of the assembly can be further downsized by thisdesign.

The recess 53 forms a drive portion for a tool such as a screw driver.It is not restricted to the shown star-like contour but can also haveother shapes such as, for example, a polygon, a wavy or a flower-likecontour or can have interruptions so that only groove portions orrecesses are provided.

With the design described above a large pivot angle such that a range ofmotion of at least 60° up to 70° can be achieved.

Next, the second holes 3 are described with reference to FIGS. 1, 8 and9. The second holes 3 have a substantially oblong shape with the longside oriented in the direction of the longitudinal axis L. As can beseen in particular in FIG. 8, a hole 3 has an upper seat portion 31which has the shape of a hollow spherical section matching the exteriorspherical surface portion of the head 42. Further, a lower seat portion32 is provided at the opposite short side of the oblong hole 3 adjacentthe bottom side 1 b. The shape of the seat portion 32 is also sphericaland matches the shape of the exterior surface portion of the head 42.Between the upper seat portion 31 and the lower seat portion 32 aninclined spherical groove 33 extends along the inner wall of the longside of the oblong hole, the size of which is adapted to the exteriorsurface portion of the head 42 so that the head 42 can be guided alongthe groove 33.

The elements of the bone plate assembly are made of a body compatiblematerial, such as a body compatible metal, for example stainless steelor titanium or a body compatible metal alloy such as Ni—Ti alloys, forexample Nitinol, or of a body compatible plastic material, for examplemedical grade PEEK or of combinations thereof. For example, the platemember, the locking elements and the bone anchors can be made ofdifferent materials.

Now, use of the bone plate assembly according to the first embodimentwill be described. First, the number of bone screws necessary for thestabilization of the bone parts or bone fragments is determined. Itshould be noted that in some cases it is not necessary to use all theholes provided on the plate member. Holes which are not used for bonescrews may be closed by a plug member to be described below. Once, thenecessary number and types of the screws are determined, the screws areinserted into the first holes 2 and/or second holes 3. After positioningthe plate member at the fracture site the screws are inserted into thebone parts at the desired angle. The bone screws are inserted into theholes at this desired angle and the spherical seat allows placement ofthe head of the screw in the hole at this angle. After full insertion ofbone screws the head 42 of each bone screw, which is inserted into afirst hole 2, abuts against the seat portion 21 of the hole 2. Alreadyin this condition the angle between the bone screw and the plate memberis fixed when at least two bone screws are inserted. To furtherstabilize the connection between the bone screw and the plate thelocking element 5 can be used which is inserted into the bore andtightened so that it locks the head in the selected angular position. Ifdesired, different locking elements can be applied to different screwsin order to provide for full locking, frictional locking or freeangulation with the locking member only preventing pull-out of the screwas described above.

The holes 3 are used for providing a self-compression effect. The bonescrews 4 which are inserted into the oblong holes 3 are orientedsubstantially vertically relative to the plate member. The screw head 42which is first seated in the upper seat portion 31 slides downwardsalong the inclined groove 33 until it rests in the lower seat portion32. Preferably, the second holes are arranged such that the lower seatportion 32 faces the center of the plate member. Since the screw head 42is fixedly connected to the threaded shank 41 which is screwed into thebone the movement of the head 42 along the groove 33 leads to a movementof the bone part or fragment relative the plate and as a result to acompression of the bone parts against each other and against the platemember.

A second embodiment will be described with reference to FIGS. 12 to 14.Parts which are the same as those of the previous embodiments aredesignated with the same reference numerals and the description thereofis not repeated. As can be seen in FIG. 12 the plate member 1′ has acontoured shape. The outer contour in a top view is slightly trapezoidalwith rounded corners and a broadened middle portion. Further,spherically shaped recesses 7 are provided at the lower side 1 b toreduce the contact area with the bone. A minimal contact area of theplate member with the bone surface may be required in certain cases toprevent injury of the bone surface and compression of blood vessels etc.

The recesses 7 also result in plate member portions with differentthickness. The plate can be bent in portions with a smaller thicknessthe bone plate assembly can be used as a dynamic compression plate.

In this embodiment, two holes are provided at the short sides is andthree holes are provided in the center of the plate in a longitudinaldirection. The holes 2 may be offset from the longitudinal axis. Thenumber is not limited to the number shown but can vary. Between theholes 2 near the short sides 1 c and the holes 2 in the center secondholes 3 may be provided for self-compression.

In the case of small bone fragments which shall be immobilized by meansof the bone plate assembly not all holes may be used for bone screws 4.As shown in FIGS. 12 and 13, for example, the first holes 2 in thecenter section of the plate member are not used for screws. They may beclosed by a plug member 8 shown in FIG. 14. The plug member 8 issubstantially cylindrical with a top side 8 a and a bottom side 8 b. Ithas a first portion 81 which has a threaded portion 82 cooperating withthe threaded portion 23 of the bore 22. The first portion 81 fits intothe bore 22 so that the top surface 8 a is substantially flush with thetop surface 1 a of the plate member 12′. A recess 53 which has anapproximate star-shape is provided in the top surface 8 a of the plugmember as for the locking element 5. The plug member can preventcracking or breaking of a plate member at the thinned portions 7.

Adjacent the cylindrical portion 81 the plug member 8 has a sphericallyshaped projection 83 which fits into the seat portion 21 of the hole 2so that after insertion of the plug member 8 the hole 2 is closed. Thebottom side 8 d may be flush with the bottom side 1 b of the platemember.

Alternatively, a locking element as described above can be used as aplug member.

In a further embodiment, the bone plate assembly includes a modifiedlocking element as shown in FIGS. 15 a), 15 b). The locking element 5′differs from the locking element of the previous embodiments by theshape of the recess 53′″ at the top side 5 a of the locking elementwhich serves for the driver. Further a coaxial through-hole 54 isprovided. All other features of the locking element are the same or canbe present in the same manner as for the locking element of the previousembodiments. The coaxial through-hole 54 extends from the top side 5 athrough the locking element and continuous in the recess 52. Thediameter of the coaxial through-hole is configured. Hence, the diameterof the through-hole can be larger than the diameter of the free end 42 bof the screw head 42. This enables the screw head 42 to at least partlyextend into the through-hole 54. The coaxial through hole 54 allows tofurther reduce the thickness of the plate member and the lockingelement, since the head of the screw can partially extend into the holewhen the screw is in an angled position as shown in FIG. 15 b)

The recess for the driver which is used to tighten the locking element5′ is in this embodiment composed of a plurality of spaced apartrecesses forming elongate pockets 53′″ which are arranged around thecentral axis of the locking element in a circumferential manner.

The depth of the pockets 53′″ is configured such that the pockets extendbelow the free end 42 b of the screw head when the screw head 42 restsin the seat 21. In other words, the distance b of the deepest portion ofthe pockets 53″ from the top side 1 a is larger than the deepest portionof the recess 52 from the top side 1 a in the previous embodiments.Since the recess 53″ extends into the body of the locking element in anarea around the screw head the locking element has a small thickness.Therefore, a low profile polyaxial locking plate is provided. In amodified embodiment the recess may extend completely through the lockingelement.

In a further modification shown in FIG. 16, the locking element 5″ hasan annular rim 55 on the top side 5 a.

In FIGS. 17 a) to 17 e) schematic sectional views of a portion of thebone plate assembly with a polyaxial bone screw and a locking elementare shown that explain a dimensional relationship of the recess for thedriver and the head of the bone screw. In FIG. 17 a) a locking elementis shown wherein the recess 52 for the head extends into the top side 5a so that a through-hole 54 is formed. A first level L1 is defined bythe highest point of a circle that is drawn around the screw head with adiameter that corresponds to the largest diameter d of the screw head.The largest diameter d of the screw head 42 is located above the opening20 on the bottom side of the bone plate. The bottom of the recess 53′″for the driver defines a second level L2. As shown in FIG. 17 a), thelevel L2 is below L1. With this relationship the bone plate assembly hasa low profile. Simultaneously, a large range of angulation of the screwcan be achieved.

In FIG. 17 b) the locking element is similar to the locking elementshown in FIG. 16 and has an annular rim 55 at the top side. Also in thiscase, the highest point of a circle which is projected from the positionof the greatest diameter d of the screw head defines a level L1 and thebottom of the recess 53′″ defines a level L2 which is below L1.

FIG. 17 c) shows a locking element that has the highest point of acircle projected from the position with the largest diameter d of thescrew head 42 on a level L1 which is on the same level L2 as the bottomof the recess 53′″ for the driver. A low profile can still be obtained.

In FIG. 17 d) the top surface 5 a of the locking element is closed. Thehighest point of the circle is above the closed top surface. Also inthis case the level L2 is below

FIG. 17 e) shows a locking element which has a conically-shaped firstrecess 52′ which contacts the head 42 of the polyaxial bone screw 4.Also in this case the level L2 is the same as the level L1 or below L1.

The contact area between the screw head 42 and the recess 52 is notlimited to a spherical surface, it can be a line contact or can haveanother shape. Thus, the recess 52 for the screw head is not limited toa spherical or a conical form but can have various shapes.

The screw head needs not to be completely spherical. It is sufficientthat it has a spherically shaped surface portion. It can also be shapedotherwise, as long as it can pivot in the seat.

FIGS. 18 to 20 show a tool for use with locking element and the boneplate assembly. The tool 60 comprises a driver 61 with an engagementportion 62 for engaging the locking element and a handle 63. Theengagement portion 62 of the driver 61 is configured to engage theengagement structure such as the pockets 53′″ in the locking element 5′of FIGS. 15 and 16 or the groove 53 in the locking element 5 of FIGS. 1to 9. The driver 61 extends with its engagement portion 62 through atubular holder 64 and is axially movable and rotatable therein. Theholder 64 has at a distance from its free end which faces away from thehandle 63 a radial slot 65 which is configured to pass the lockingelement 5 therethrough. Further, the counter holder 64 has at its freeend a portion with an internal thread 66 cooperating with the externalthread 51 of the locking element 5.

The steps of taking up the locking element with the tool 60 areschematically shown in FIGS. 21 a) to 21 d). Exemplary the lockingelement of FIGS. 15 and 16 is shown. In a first step, the driver 61 isretracted and the holder 64 is free at the location of the slot 65. In anext step as shown in FIG. 21 b) the locking element 5′ is introducedthrough the slot 65 into the holder 64. The bottom side 5 b of thelocking element, which has the recess 52 for the head, faces thethreaded portion 66 of the holder 64. Then, as shown in FIG. 21 c), thedriver 61 is moved through the holder 64 to engage the pockets 53′″ andthe locking element 5′ is screwed into the threaded portion 66 of theholder 64. Then, as shown in FIG. 21 d), the driver 61 can be retractedand the locking element can be placed into the hole 2. Finally, as shownin FIG. 20 before, the driver 61 can be advanced again and engage thepockets 53′″ of the locking element to screw the locking element furtherinto the hole 2. If desired, the locking element is tightened to pressonto the head to fix the angular position of the bone screw.

With the tool, the locking element can be safely taken up and held inthe holder. In addition, it can be safely placed into the hole andinserted therein.

Referring again to the locking element, it may be configured to be usedwith a polyaxial bone anchor, and be substantially cylindrical with abottom side (5 b) facing a head (42) of the bone anchor, a top side (5a) opposite to the bottom side (5 b), and a surface portion with anexternal thread. The locking element may have a first recess (52, 52′)at the bottom side for accommodating at least a portion of the head(42), and a second recess (53, 53′, 53′″) at the top side (5 a) forengagement with a driver, the second recess being located outside thecenter of the top side (5 a). The head (42) may have a portion with alargest diameter (d), and a first level L1 defined by the highest pointof a circle with the largest diameter drawn around the head, and asecond level L2 is defined by the bottom of the second recess (53, 53′,53″), L2 being at a level not higher than L1.

The locking element may include a coaxial through-hole (54). A contactarea of the first recess (52) that contacts the head may be a conicallyshaped contact, a line contact, and/or a spherically-shaped contact. Thesecond recess (53, 53″, 53″) may be located substantially in a radiallyouter area of the top side (5 a). The second recess (53, 53′) mayinclude a ring-shaped groove with a contour configured have form-fittingengagement with the driver. The contour may be selected from at leastone of the group consisting of a star-like contour, a polygonal contour,and a flower-like contour. The second recess may include a plurality ofpockets (53″). The pockets may be configured to extend through thelocking element into the bottom side (5 b). A distance (a) of a deepestportion of the second recess (52), measured from the top side (5 a) issmaller than a depth (b) of the first recess (53, 53′, 53″) measuredfrom the top side (1 a). The locking element may also include a portion(51) with an external thread. The locking element (5) may include aportion with the external thread that spans the entire locking element(5), making the locking element (5) fully threaded, and/or a portionwith the external thread that does not span the entire locking element(5), having a non-threaded section (51 a) adjacent the bottom side (5b). An axial length of the non-threaded section (51 a) may besubstantially equal to or smaller than the axial length of a threadedsection of the portion with the external thread (51).

Referring again to a bone plate assembly, it may include an elongateplate member (1, 1′) with a top side (1 a) and a bottom side (1 b), atleast two holes (2, 2, 3) extending from the top side to the bottomside, and at least two bone anchors (4), each bone anchor comprising ashank (41) configured to anchor into the bone, and a head (42) having aportion with a largest outer diameter (d), wherein the shank isextendable through the hole. At least one of the holes (2) may beconfigured to define a seat (21) at the bottom side (1 b) adapted topivotably receive the head (42), wherein the at least one of the holes(2) includes a bore (22) at the top side (1 a) with a bore axis and aninner diameter (D) at least as large as the largest diameter (d) of thehead (42). The locking element (5, 5′, 5″, 5′″) may be configured to beinserted into the bore (22), having a central axis coaxial with the boreaxis, a bottom side (5 b) facing the head, and a top side (5 a) facingaway from the head. The locking element may have a first recess (52,52′) at the bottom side for at least partially covering the head, and asecond recess (53, 53′, 53′″) at the top side (5 a) for engagement witha driver, the second recess being located outside the center of the topside (5 a). The highest point of a circle with the largest diameter (d)that is drawn around the head may define a first level L1 and the bottomof the second recess (53, 53′, 53′″) may define a second level L2, whichis at a level not higher than L1.

The bone plate assembly may include a plurality of locking elements andthese may be configured to lock the head and/or prevent loosening of thebone anchoring element and/or bone plate assembly, while allowingpivotal movement.

Referring once again to a bone plate assembly, it may include anelongate plate member (1, 1′) with a top side (1 a) and a bottom side (1b), at least two holes (2, 2, 3) extending from the top side to thebottom side, and at least two bone anchors (4), each bone anchorcomprising a shank (41) configured to anchor into the bone, and a head(42) having a top (42 a) and a portion with a largest outer diameter(d), wherein the shank is extendable through the hole. At least one ofthe holes (2) may be configured to define a seat (21) at the bottom side(1 b) adapted to pivotably receive the head, (42) wherein the at leastone of the holes includes a bore (22) at the top side (1 a) with a boreaxis and an inner diameter (D) which is at least as large as the largestdiameter (d) of the head (42). It shall be understood by one of ordinaryskill in the art, that the head may be pivotably received when a boneanchor is inserted into the bone plate assembly, for example. However,at such times as, for example, upon insertion of the bone anchor intobone, pivoting may be restricted. A locking element (5, 5′) isconfigured to be inserted into the bore and having a central axiscoaxial with the bore axis, a bottom side (5 b) facing the head, a topside (5 a) facing away from the head and a recess (53, 53′, 53′″) at thetop side (5 a) configured to be engaged with a driver, the recess beinglocated outside the center of the top side (5 a) and having a depth thatextends below the top (42 a) of the screw head (42) when the screw head(42) is in the seat (21).

The bore may have a portion (23) with an internal thread and the lockingelement (5) may have a portion (51) with an external thread adapted tocooperate with the internal thread of the bore.

At least one of the internal thread and the external thread may includea two-start thread. Furthermore, another locking element may be includedin the bone plate assembly, and the locking element and the anotherlocking element may have different axial lengths. The first recess (52)may include a spherical shape and a depth measured from the bottom side(5 b) substantially equal to or slightly larger than half of thediameter (d) of the head. The bone plate assembly may also include aplug member (8) adapted to close a hole if a bone anchor is not used.Referring now to a tool adapted for use with a bone plate assemblydescribed herein, it may include a driver (61) with an engagementportion (62) configured to engage a locking element; and a tubularholder (64). The driver may be configured to extend with the engagementportion (62) through the tubular holder (64), and may be axially movableand rotatable within the holder. The holder may have a radial slot (65)adapted for insertion of the locking element. The holder may include aportion with an internal thread (66) for cooperation with an externalthread (51) of the locking element.

The shape of the plate member is not restricted to the embodimentsshown. Other shapes are also conceivable. The bone anchor is not limitedto a bone screw which has a threaded shank. Smooth, barbed or roughenedpins are also conceivable. Further, any known bone screws may be used.Additionally, other bone anchors may be used.

1. A locking element configured to be used with a polyaxial bone anchor,the locking element being substantially cylindrical with a bottom sidefacing a head of the bone anchor, a top side opposite to the bottomside, and a surface portion with an external thread; wherein, thelocking element has a first recess at the bottom side for accommodatingat least a portion of the head, and a second recess at the top side forengagement with a driver, the second recess being located outside thecenter of the top side, and wherein the head has a portion with alargest diameter (d), and a first level L1 is defined by the highestpoint of a circle with the largest diameter drawn around the head, and asecond level L2 is defined by the bottom of the second recess, L2 beingat a level not higher than L1.
 2. The locking element of claim 1,further comprising a coaxial through-hole.
 3. The locking element ofclaim 1, wherein a contact area of the first recess that contacts thehead comprises at least one of a conically shaped contact, a linecontact, and a spherically-shaped contact.
 4. (canceled)
 5. The lockingelement of claim 1, wherein the second recess is located substantiallyin a radially outer area of the top side.
 6. The locking element ofclaim 5, wherein the second recess comprises a ring-shaped groove with acontour configured to have form-fitting engagement with the driver.7-11. (canceled)
 12. The locking element of claim 1, wherein the lockingelement is configured according to one of: the portion with the externalthread spans the entire locking element, making the locking elementfully threaded, and the portion with the external thread does not spanthe entire locking element, having a non-threaded section adjacent thebottom side.
 13. The locking element of claim 12, wherein an axiallength of the non-threaded section is substantially equal to or smallerthan the axial length of a threaded section of the portion with theexternal thread. 14-27. (canceled)
 28. A bone plate assembly,comprising: an elongate plate member with a top side and a bottom side,at least two holes extending from the top side to the bottom side, atleast two bone anchors, each bone anchor comprising a shank configuredto anchor into the bone, and a head having a top and a portion with alargest outer diameter (d), wherein the shank is extendable through thehole; wherein at least one of the holes is configured to define a seatat the bottom side adapted to pivotably receive the head and wherein theat least one of the holes includes a bore at the top side with a boreaxis and an inner diameter (D) which is at least as large as the largestdiameter (d) of the head; and a locking element configured to beinserted into the bore and having a central axis coaxial with the boreaxis, a bottom side facing the head, a top side facing away from thehead and a recess at the top side configured to be engaged with adriver, the recess being located outside the center of the top side andhaving a depth that extends below the top of the screw head when thescrew head is in the seat.
 29. The bone plate assembly of claim 28,wherein the recess is located substantially in a radially outer area ofthe top side.
 30. The bone plate assembly of claim 29, wherein thesecond recess comprises a ring-shaped groove with a contour configuredhave form-fitting engagement with the driver. 31-32. (canceled)
 33. Thebone plate assembly of claim 28, wherein the locking element has asecond recess at the bottom side which is configured to at leastpartially cover the head.
 34. The bone plate assembly of one of claim33, wherein the locking element further comprises a coaxialthrough-hole.
 35. The bone plate assembly of claim 28, wherein the borehas a portion with an internal thread and the locking element has aportion with an external thread adapted to cooperate with the internalthread of the bore.
 36. The bone plate assembly of claim 35, wherein atleast one of the internal thread and the external thread comprises atwo-start thread.
 37. The bone plate assembly of claim 28, furthercomprising another locking element, wherein the locking element and theanother locking element have different axial lengths.
 38. The bone plateassembly of claim 28, wherein the first recess comprises a sphericalshape and a depth measured from the bottom side substantially equal toor slightly larger than half of the diameter (d) of the head.
 39. Thebone plate assembly of claim 28, further comprising a plug memberadapted to close a hole if a bone anchor is not used.
 40. A tool adaptedfor use with a bone plate assembly, comprising: a driver with anengagement portion configured to engage a locking element; and a tubularholder; wherein the driver is configured to extend with the engagementportion through the tubular holder, wherein the driver is axiallymovable and rotatable within the holder, and wherein the holder has aradial slot adapted for insertion of the locking element, and wherein,the bone plate assembly comprises an elongate plate member with a topside and a bottom side, at least two holes extending from the top sideto the bottom side, at least two bone anchors, each bone anchorcomprising a shank configured to anchor into the bone, and a head havinga top and a portion with a largest outer diameter (d), wherein the shankis extendable through the hole; wherein at least one of the holes isconfigured to define a seat at the bottom side adapted to pivotablyreceive the head and wherein the at least one of the holes includes abore at the top side with a bore axis and an inner diameter (D) which isat least as large as the largest diameter (d) of the head; and a lockingelement configured to be inserted into the bore and having a centralaxis coaxial with the bore axis, a bottom side facing the head, a topside facing away from the head and a recess at the top side configuredto be engaged with a driver, the recess being located outside the centerof the top side and having a depth that extends below the top of thescrew head when the screw head is in the seat.
 41. The tool of claim 40,wherein the holder comprises a portion with an internal thread forcooperation with an external thread of the locking element.